If you have obstructed someone mistakenly or altered your mind about the block, however, it is likewise reversible. You can alter your Personal privacy settings to unclog and enable someone back into your Facebook world.
Unblock Someone On Facebook
1. Click the down arrow next to "Account" at the top right of any page on Facebook.
2. Click "Privacy Preferences" from the Account menu.
3. Click the "Edit Your Lists" link under the Block Lists heading. It is at the bottom center of the page.
4. Click the "Unclog" link next to the name of the person you want to unclog. A list of individuals you've blocked is found under the Block Users section.
Suggestion
Unblocking someone does not immediately put that person back in your good friend list. You should add the newly unblocked person as a friend and await them to verify the request.
And one more thing please don’t forget to share this awesome trick to use the Unblock Someone On Facebook with your friends.